Modem for communicating at high speed over voice-grade telephone circuits

摘要

A modem for connecting data terminal equipment (DTE) to a remote DTE via a the general switched telephone network or leased lines at data rates of 300, 1200 and 2400 bps using standard modulation techniques, and additionally providing virtual full duplex transmission capability at 9600 bps using trellis code modulation (TCM). The high-speed 9600 bps path is implemented by asymmetrical frequency division of the available bandwidth into a high speed, wideband forward channel (9600 bps) and a low speed, narrowband backchannel (300 bps). The high speed transmitting channel is assigned to that modem having the greatest data demand and the direction is dynamically reversed whenever the amount of data awaiting transmission over the low speed channel exceeds a predetermined maximum backlog, provided that a direction reversal has not taken place for at least a specified minimum interval during which use of the forward channel is guaranteed. The transmission speed of the high speed channel is adaptively adjusted in response to changes in the quality of the data link provided by the dial up telephone facility. Cyclic redundancy checking is used to monitor error rates and failed retraining attempts are also used to indicate the need for lower transmission speeds. The trellis decoding algorithm generates a metric value which is used during high speed transmission to indicate when a speed up should be attempted, and the equalization eyescatter signal is used for the same purpose when transmission is taking place at 4800 baud when trellis coding is not used. Rate change requests are sent over the low speed backchannel to the high speed channel which then executes a mandatory speed switch.